1. Field of the Invention
The present invention relates to a centrifugal compressor to be applied, for example, as a supercharger of an engine.
2. Description of the Prior Art
FIG. 3 shows a longitudinal section of a centrifugal compressor of the prior art, and FIG. 4 illustrates the operation of the same. A labyrinth packing 5 is sandwiched between the back of an impeller 1 of the centrifugal compressor and a casing 12. The labyrinth packing 5 seals a space 7 by being located between the outlet and back of the impeller 1. The space 7 is vented to the outside through a ventilation hole 8 for releasing a slight amount of air, which comes into the space 7 along the labyrinth packing 5, to the outside to thereby decrease the pressure in the space 7. As a result, the pressurized air leaking from the outlet of the impeller 1 to the back of the impeller 1 is prevented from establishing a thrust pushing a rotor shaft 2 in a direction toward the inlet of the impeller 1. Thus, the facial pressure on a main thrust bearing 3 is lowered to lighten the load upon the main thrust bearing 3. Reference numerals 11 and 4 designate a thrust collar and a thrust bearing, respectively.
In the existing centrifugal compressor described above, the air temperature at the outlet of the impeller 1 becomes higher with a higher pressure ratio of the centrifugal compressor. Even when air at the room temperature is aspirated, for example, its temperature reaches 200.degree. C. or more at the outlet of the impeller 1 if the pressure ratio is about 4.0. This hot air is additionally heated, while passing through the labyrinth packing 5, by the heat of friction which is produced by the rotations of the fins of the labyrinth packing 5. Thus, the back of the impeller 1 is heated. The centrifugal compressor of this single stage type intaking the atmospheric air usually has its impeller made of an aluminum alloy, and the impeller 1 is degraded due to its temperature rise when heated by the air at its back. This makes it difficult to retain a high pressure ratio if the impeller is made of an aluminum alloy. The temperature rise of the impeller 1 in turn leads to a rise in the air temperature, thus causing an efficiency drop of the compressor.